The present invention relates to an improvement in an automatic tracking system for constantly orienting an antenna device mounted for satellite communications on a swaying body toward a satellite and, more particularly, to an automatic tracking system for a so-called AZ-EL mount antenna which supports the antenna in vertical and horizontal axes. The swaying body may comprise a platform floating on the sea for the production of petroleum or gas, such as an oil rig.
In an antenna device for satellite communications mounted on a platform of the kind concerned, use is made of an automatic tracking system which automatically keeps the main beam direction of the antenna directed toward a satellite. Such a tracking system is based on so-called hill-climb tracking in which an antenna support is driven to make the receive level from the satellite maximum in an intermittent fashion, that is repeating operation and suspension alternately at predetermined intervals.
As the platform loaded with the step tracking antenna device is caused to sway in any direction, the receive level from the satellite is lowered. Various implementations have heretofore been proposed to make up for the decrease in receive level. One of them employs continuous step tracking instead of the intermittent one, that is, constantly controls the drive of the antenna by the operation and suspension at predetermined intervals and eliminating the intervals between sequential tracking operations. Another is to use a vertical gyro to allow pitching and rolling angles due to the movement of the platform for coordinate conversion, so that the change in angle may be compensated for with respect to each of azimuth (AZ) and elevation (EL) axes while performing continuous tracking.
The constant drive type system, however, causes mechanical portions to wear fast due to the continuous drive and thereby limits the service life of the system. Another drawback is that the continuous drive wastes power for driving motors which are adapted to drive the antenna. The vertical gyro type system, in addition to the limited life due to continuous drive, has various drawbacks such as poor reliability for a long time of operation and the need for an expensive vertical gyro and an intricate coordinate conversion device.